The rapid growth of commercial aviation has put immense pressure on the air traffic control system. Air traffic controller mental workload has already become the limiting factor in attempts to increase capacity in en route airspace. The concept of Free Flight has been proposed to increase capacity in the airspace. Under Free Flight, air traffic controllers will no longer actively control the airspace, but rather passively monitor the airspace, in order to intervene in emergencies. Because controllers have difficulties to intervene from their passive role, automated decision aids were proposed to improve their performance. The effects of Free Flight and automated decision aids for the detection of conflicts on controller performance and mental workload were investigated experimentally in three simulations. Professional air traffic controllers served as participants. In addition to performance measures, eye movement data was collected. Results showed that automation could improve controller workload and reduce mental workload as long as the automation functioned reliably. However, controllers indicated better performance under manual than under automated condition when the automation became unreliable. This could be explained with the eye movement data. Implications for the design of automated systems were discussed.